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Yeah, it sounds like you are attempting to send the transaction with too low of a fee.

You should double check that Dynamic Fees are enabled in the Preferences as this should likely fix the issue as mentioned by OmegaStarScream. You might also want to upgrade to 2.8.2 which is a slightly newer version than you are currently using.

Yeah, it sounds like you are attempting to send the transaction with too low of a fee.

Coinb.in spits out that same error message when you try to broadcast a raw transaction with an extremely low fee. The screenshot below was from when I tried broadcasting someone's transaction that only paid 1 sat/B. Maybe coinb.in uses an electrum server in its backend.